Adaptive equalization and synchronization

On Sun, 16 Jun 2013 07:37:00 -0700, Eric Weaver <weav@sigma.net>
wrote:

>
>EJ;
>
>Just out of curiosity, can it work to always start with a known training 
>symbol (or two or three), come up with a "deconvolution kernel" that 
>would transform the found one closer to the expected one, and use that 
>for the whole message block?

Yes.   Many systems do exactly that.   The training sequence may be
significantly longer than two or three symbols depending on the
expected channels and how much noise reduction you need in the
estimate, but the idea is the same.

The other assumption is that the channel is static, or close to it,
during the length of the transmission.   This is often a reasonable
assumption if the burst isn't huge since the channel coherence time is
usually very long compared to a usefully long burst.

>I suppose it depends on the mod scheme in question.

It's used in many different systems, including OFDM.  802.11 works
this way.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

>>On Mon, 03 Jun 2013 06:48:26 -0500, DougB wrote:
>>
>>>>>Only a fractionally spaced equalizer will resolve timing phase.  If
>you
>>> use
>>>>>a T spaced equalizer you had better have good symbol timing recovery
>>> prior
>>>>>to the equalizer.  Carrier reference can be resolved after
>equalization
>>>>>unless the frequency offset is large in which case you can split the
>>> loop
>>>>>and recover frequency prior to the equalizer and phase/remaining
>>> frequency
>>>>>after the equalizer.
>>>>>-Doug
>>>>
>>>>
>>>>One of the disadvantages of using the EQ to correct timing is that it
>>>>consumes degrees of freedom usually intended for combating the
channel
>>>>distortion.   Those degrees of freedom cost complexity, and usually
the
>>>>complexity of the EQ is much greater than the complexity of a decent
>>>>timing recovery system.   So, in my experience, the systems that try
to
>>>>use the EQ for synchronization or to aid in synchronization wind up
>with
>>>>a heavier complexity load than alternative methods.  Separating the
two
>>>>tasks can be advantageous for the performance of both the EQ and the
>>>>timing recovery as well as complexity.
>>>>
>>>>
>>>>
>>>>Eric Jacobsen
>>>>Anchor Hill Communications
>>>>http://www.anchorhill.com
>>>>
>>>>
>>> That's true, but if the channel highly distorted, there is no TED
that
>>> is going to give you the optimum timing phase - that can only be
gotten
>>> from a fractionally spaced equalizer.  Still use timing recovery as
>>> normal, but let the FSE optimize the timing phase - it can synthesize
>>> the proper fractional delay.
>>
>>I considered just letting the equalizer take care of delay.  It would 
>>certainly make my life easier in many respects.
>>
>>My problem with this system is that I can't count on a perfect match 
>>between transmit and receive clocks, so the change in delay from the 
>>start of a run to the end would require an equalizer with an unfeasible

>>amount of delay.  Hence, I seek a reliable way to synchronize that does

>>not leave the equalizer holding the bag.
>>
>>-- 
>>My liberal friends think I'm a conservative kook.
>>My conservative friends think I'm a liberal kook.
>>Why am I not happy that they have found common ground?
>>
>>Tim Wescott, Communications, Control, Circuits & Software
>>http://www.wescottdesign.com
>>
>
>In the given scenario, in my opinion, the TED is not even supposed to
lock
>onto the timing phase; it will lock to the symbol rate (or transmit
clock).
>So any NDA TED like Gardner followed by a fractionally spaced equalizer
>should do the job for you. A very interesting and practical reference
paper
>is the one by John Treichler, "Practical blind demodulators for
high-order
>QAM signals" published in 1998. The most important part I think is the
>fractional tap-spacing because a symbol rate equalizer can easily lose
the
>battle due to aliasing.

Hello, I have read the whole discussion, it is very interesting. I have
got two simple questions:
- If the synchronization is placed before a T/2-FSE, the output of the
interpolation unit is only to be fed to the TED? I mean, the signal is not
resampled at this point, because I still want 2 samples per symbol at the
input of the equalizer, isnt it?
- If the synchronization is placed after equalization, and, thus, the
input signal is at 1sample/symbol, must I use a synchronization algorithm
which operates at this sample rate (1sample/symbol)? For example M&M?




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